A wire electric discharge machining apparatus is an apparatus that generates a pulse-like electric discharge between a wire serving as an electrode and a work, which are arranged to be opposed to each other in the water, and machines the work into a desired shape using thermal energy of the electric discharge. In the wire electric discharge machining apparatus, the wire is subjected to a reaction force in a direction opposite to a discharge direction after the electric discharge ends. To eliminate discharge machining scrap, in general, high-pressure machining fluid is spayed from positions above and below a position where the wire is opposed to the work to the opposed position. The wire is vibrated by the discharge reaction force, the spray of the machining fluid, and the like. As a result, straightness accuracy of the work is easily deteriorated.
To realize the straightness accuracy of the work, parameters such as machining energy, machining speed, wire tension, and machining fluid pressure are optimally selected for first cut to fourth or fifth cut and the parameters are merged into a condition table for each work material, board thickness, and wire type. However, considerable time and labor are required for creation of the condition table.
In the wire electric discharge machining apparatus, a method of directly connecting the work to one electrode of a machining power supply and connecting the traveling wire to the other electrode of the machining power supply via feeding points, which are in sliding contact with the wire, is adopted for supplying power between the electrodes. In general, the feeding points are provided in two places above and below the position where the wire is opposed to the work. Thus, discharge positions are detected using this structure. In the wire, flow paths of discharge currents are present in parallel on an upper side and a lower side thereof. Since the wire is a resistor and resistance thereof is proportional to a wire length, it is possible to determine the discharge positions according to a flow-dividing ratio of a parallel circuit. There are various methods of calculating the discharge positions such as a method of using a differential value, a method of observing a supply voltage, and a method of calculating the differential value and the supply voltage. For example, a method of detecting a current difference caused by a difference in a resistance ratio using a current sensor and specifying a position as disclosed in a Patent Document 1 may be considered a general method.
One of causes of breaking of wire in the wire electric discharge machining apparatus is “concentrated discharge” in which electric discharges are concentrated in one place of wire. If the detection of discharge positions makes it possible to determine that electric discharges are concentrated in an identical place of the wire, it is possible to reduce the machining energy to facilitate prevention of breaking of wire. It is also possible to change machining conditions such as wire tension and a machining liquid pressure to prevent the concentrated discharge.
Concerning this point, in the Patent Document 1, a method of changing, when a concentrated discharge is detected, traveling speed of the wire to agitate machining scrap and changing a position of a discharge point according to the change in the traveling speed to solve over-concentration of electric discharges is adopted.
In the Patent Document 1, a discharge state in an extremely short period is monitored in this way. However, as other methods of utilizing the discharge position detection, for example, as disclosed in a Patent Document 2, it is also possible to grasp a state of an entire electric discharge in a relatively long-term range and determine a machining board thickness, a machining shape, and the like.
Conventionally, in automatically machining a work, a machining shape of which changes complicatedly, it is necessary to input a place where a shape of the work changes in a machining program in advance to change machining conditions or set machining conditions with low machining energy for preventing breaking of wire from occurring in an entire machining range. This method is inefficient and low in productivity.
Thus, in the Patent Document 2, in addition to the discharge-position determining means in the Patent Document 1, machining-area recognizing means for recognizing a history of change in a machining shape is provided to recognize, even if the machining shape (a board thickness) changes complicatedly, the change and automatically select optimum machining conditions.
However, although it is possible to measure discharge positions between the wire and the work as described above, it is impossible to induce an electric discharge in an arbitrary position. In other words, the main purpose of the Patent Documents 1 and 2 is to measure discharge positions. It is impossible to positively control electric discharges in the discharge positions measured. To make use of the measured discharge positions for the control, it is necessary to output, after statistically processing a plurality of electric discharges, a calculation result as a control signal.
In short, conventionally, shape machining in general wire electric discharge machining utilizes an X-axis and a Y-axis that define a plane perpendicular to wire. A Z-axis, which is in a traveling direction of the wire, is only set initially depending on a board thickness of a work. It is impossible to perform the shape machining in the Z-axis direction.
In a non-Patent Literature 1, machining characteristics at the time when an electric discharge occurs only once and in a continuous electric discharge are explained. In a non-Patent Literature 2, a relation between a discharge position and a discharge voltage in the case of a large current discharge is reported.
Patent Document 1: Japanese Patent No. 287968
Patent Document 2: Japanese Patent No. 3085040
Non-Patent Literature 1: “Discharge Machining Technology”, the Nikkan Kogyo Shimbun, page 25
Non-Patent Literature 2: “Study of Breaking-wire Prevention Control for Wire electric discharge machining (Third Report)” Japan Society of Electrical-Machining Engineers Magazine, vol. 36, No. 81 (2002)